The present invention relates to high frequency antenna systems and more particularly to microwave and millimeter wave antennas.
Interference to microwave and millimeter-wave signals is often caused by higher-than-desired signal levels outside the main antenna-beam region, particularly in what are generally termed the backlobe and far-out sidelobe regions of the antenna pattern. An antenna which could minimize the signal-level in these regions has been long sought after, yet prior designs have been unsatisfactory.
Pyramidal-horn reflector systems are of long standing use in the major microwave radio-relay networks. Although this type of antenna has lower signal-levels in the far-out sidelobe and backlobe regions than most other antenna types, it is characterized by unnecessarily high such levels. Furthermore, prior antenna types of this nature are also characterized by relatively high "spillover"--radiation lobes caused by direct radiation from the feed-horn of energy which misses the reflector.
Prior attempts to minimize "spillover" have resulted in conical-horn reflector systems in which a cylindrical shield is fitted around the feed-horn aperture (described for example in "The Electrical Characteristics of the Conical Horn Reflector Antenna"; J. M. Hines, Tingye Li, and R. H. Turrin, Bell System Technical Journal, Vol. XLII, July 1963, No. 4, part 1, pp. 1187-1211). Unfortunately however, this antenna type suffers from relatively high signal-levels in far-out sidelobe regions. Furthermore, this type of antenna heretofore has not employed corrugations in the interior feed-horn surface.
A third type of antenna which reduces radiation in the farout sidelobe region is the paraboloidal antenna, axially fed, which is surrounded by a cylindrical "tunnel" lined with absorbent material. This type of antenna, although satisfactory for many applications also suffers disadvantages. For example, an elevated signal-level in the sidelobe region (resulting from aperture blocking by the feed and support structure) is very undesired. Antenna systems of this nature, employing absorber materials as described, in general cannot be operated at relatively high power levels (because the absorber materials tend to become destroyed). Also, long tunnels must be fitted to the antenna in order to reduce sidelobe signal-levels at angles close to the main beam. As the length of the tunnel increases, substantial loss of gain in the signal-level of the main beam generally follows.
Corrugations have been utilized in feed-horns, but their use has generally been limited to axial feed-horns for parabolic reflectors (e.g., c.f. The scalar-feed--a high-performance feed for large paraboloid reflectors 1966, IEE Conference pub. No. 21, by A. J. Simmons and A. F. Kay). Furthermore, prior applications of corrugated feed-horns have generally been restricted to use in combination with large (in terms of wavelength) paraboloidal reflectors (c.f. Simmons and Kay, Supra), because of the high amount of aperture blocking associated with the axial feed arrangement (since aperture blocking undesirably reduces gain and causes undesirably high signal-levels in the antenna sidelobe regions).